Conveyor belt

ABSTRACT

A thermoplastic endless belt ( 100, 150 ) has a smooth outer surface ( 105 ) substantially free of discontinuities and an inner surface ( 108 ) with a plurality of teeth ( 108 ) at a given belt pitch ( 112 ). The teeth ( 108 ) are adapted to engage a sprocket ( 102, 152 ) with circumferentially spaced sheaves ( 104, 154 ) at a sprocket pitch ( 116 ) greater than the belt pitch ( 112 ). The belt ( 100, 150 ) is slightly stretchable so that the sprocket ( 102, 152 ) can drive the endless belt ( 100, 150 ) when engaging the teeth ( 108 ) within a range of load on the belt ( 100, 150 ). The belt ( 100, 150 ) is preferred for use in conveyors in food processing industries where the smooth outer surface ( 105 ) can transport food items and is easier to clean and keep free of impurities.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Application No. 60/319,133filed Mar. 5, 2002, and the benefit of International Application No.PCT/US03/03029 filed Feb. 03, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to endless belts for conveyors and, moreparticularly, to thermoplastic endless belts driven by sprockets.

2. Description of the Related Art

Low tension, direct drive conveyor belts are typically used insituations where hygiene and cleanliness are critically important. Forexample, in food processing plants such as those that process meatproducts for human consumption, low tension, direct drive belt conveyorsare used to transport items. Sanitation is critically important and,therefore, the endless belts used in such conveyors are conventionallymade of materials that can be hygienically cleaned, such asthermoplastics or stainless steel.

Known belts are typically formed of interlocking links having teeth thatare adapted to engage drive sprockets. One of the problems with suchbelts is that food particles can become lodged in the joints of theinterconnecting links. Consequently, cleaning the belts can be difficultand may require removing the belt from the conveyor system for specialcleaning operations.

One solution to this problem is the use of flexible thermoplastic beltswithout interlocking links such as that disclosed in U.S. Pat. No.5,697,491. Such belts having a smooth continuous surface (sometimescalled “homogeneous belts”) are driven by V-guides wherein a radialgroove in a drive pulley engages a longitudinal rib on the underside ofthe belt. One of the problems with such belts is that grease and oilfrom the food items can migrate to the groove or to the rib, whichcauses a loss of friction between the pulley and the belt. Consequentlythe driving force becomes unstable and unreliable. Moreover, such beltsare under tension to ensure that the pulley imparts enough drivingforce. This tensioning raises other issues beyond slippage due to oilsand contaminants. A thermoplastic belt under tension will stretch, whichmay require adjustment of the tension from time to time. In addition,there are additional costs associated with ensuring that the conveyorframe be sufficiently strong enough to handle the normal stresses of thepretensioned belt plus additional stresses caused by loading the belt.

It is known to provide a drive sprocket or drum with transverse groovesthat are complementary in shape to teeth on a flexible conveyor belt, asshown for example in U.S. Pat. No. 4,170,281. However, the belt isformed from interlocking links and the belt is still under tension. Theproblems associated with interlocking links and pretensioning remain.

Another solution is disclosed in U.S. Pat. No. 5,911,307 where a timingbelt is added to a homogeneous belt to engage a drive sprocket. As aresult, reliance upon friction for motion is minimized, and the beltneed not be under tension. There are some remaining problems, however.Assembling a timing belt to a homogeneous belt is costly and the bondingor adhering process is critical. Failure of the bond increases the riskof contamination and total belt failure.

SUMMARY OF THE INVENTION

The invention solves these and other problems by starting with aconveyor belt segment having an outer surface substantially free ofdiscontinuities and an inner surface with a plurality of teeth extendingtherefrom at a given belt pitch and opposite ends. The belt segment isstretchable through its length. The teeth are integrally formed, withthe conveyor belt segment by molding the teeth with the belt. Moldingcomprises the steps of providing a flat ribbon of thermoplasticmaterial, molding teeth onto one surface of the flat ribbon, and curingthe flat ribbon with the teeth. The belt segment can be made to aselected length and the ends spliced together to form a conveyor belt ofpredetermined length. The molding step can comprise injection molding orfriction molding.

In another aspect of the invention, a thermoplastic conveyor beltsegment has an outer surface substantially free of discontinuities andan inner surface with a plurality of teeth extending therefrom at agiven belt pitch and opposite ends. The belt segment is stretchablethrough its length. The teeth are integrally formed with the conveyorbelt segment by machining away material from the inner surface betweenadjacent teeth. The belt segment can be made to a selected length andthe ends spliced together to form a conveyor belt of predeterminedlength.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective side view of a belt according to the inventioninstalled between two sprockets;

FIG. 2 is an enlarged view in elevation of a portion of FIG. 1;

FIG. 3 is a view similar to FIG. 2 with the belt under load;

FIG. 4 is a side elevational view of a portion of the belt, partially incross-section, shown in FIGS. 1–3;

FIG. 5 is a perspective side view of a belt according to the inventioninstalled between two sprockets of a different configuration than FIG.1;

FIG. 6 is an enlarged view of a portion of FIG. 5; and

FIG. 7 is diagrammatic view illustrating a method of making a beltaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An endless belt 100 according to the invention is seen in FIG. 1 in atypical installation between two sprockets 102 and 103. The sprockets102, 103 are conventional and they can be any of a number of differentforms and sizes. One sprocket 102 is a drive sprocket, the other 103being an idler or slave sprocket. In this configuration, the upper span105 of the belt will travel in the direction of arrow 107. Each sprocket102 or 103 has a number of transverse grooves or sheaves 104 spacedaround its circumference. The belt 100 has a plurality of teeth 106equidistantly spaced from each other on the inside surface 108 of thebelt. The teeth 106 engage the sheaves 104 of each sprocket. It will beunderstood that any given installation may have one, two, or moresprockets for a single endless belt. Also, for any given installation,one or more of the sprockets may be a drive sprocket, as for examplesprocket 102, with the other sprocket or sprockets being idlersprockets. It will be understood that there need only be enough tensionon the endless belt 100 to keep at least one of its teeth 106 engaged ina sheave 104 of the drive sprocket 102.

The belt 100 has an outside surface 110 that is fairly smooth and freeof discontinuities. Preferably, the belt 100 is made of a thermoplasticmaterial such as Pebax® resin, polyester or polyurethane. The outsidesurface 110 on the upper span 105 is the carrying surface for transportof items. Because it is smooth and free of discontinuities, there is noplace for particles or contaminants to lodge. Moreover, the belt 100 canbe cleaned in situ, without the need to remove it from the installation.

Greater detail about the structure of the belt 100 is shown in FIGS. 2and 3. A combination of material selection and construction enables thebelt to stretch within a range of 0–3% of its total length under a loadranging from 0–100 pounds per inch of width (nearly 18 Kg per cm). Thebelt 100 has a pitch 112 defined as the distance between the centerlinesof adjacent teeth 106. The belt pitch 112 is measured along a belt pitchline 114, which corresponds to the neutral bending axis of the belt. Asthe belt 100 bends around the sprocket 102, the neutral bending axis isthat imaginary plane on one side of which the belt material is undercompression and on the other side of which the belt material is undertension.

Similarly, the sprocket pitch 116 is the arc length between thecenterlines of adjacent sheaves 104, measured along the sprocket's pitchcircle 118. The sprocket pitch circle 118 in this case corresponds tothe belt pitch line 114 as the belt 100 moves around the sprocket 102.In other words, the sprocket pitch circle 118 will have the same radiusas the belt pitch line 114 as the belt goes around the sprocket. For athermoplastic belt, the area of greatest stress on the belt 100 occursat zone 120, and the area of least stress occurs at zone 122 just as thebelt is released from the drive sprocket 102.

Looking now also at FIG. 4, it can be seen that each tooth 106 extendsupwardly from a surrounding inner surface 124 of the belt 100.Preferably, the belt 100 has a thickness between adjacent teeth 106 in arange of approximately 3 to 4 mm, although it will be understood thatthe actual thickness will depend upon a given application. The inventionis not limited to a particular thickness of the belt. For thisembodiment, each tooth 106 stands approximately 4 mm above the innersurface 124. Each tooth 106 has tapered sidewalls 126, each sidewallpreferably tapering from the inner surface 124 at an angle of about 72.5degrees. Each tooth will thus have a width at its base wider than thatat the tip. Each sheave 104 on the sprocket 102 is wider than thecorresponding tooth 106 on the belt 100, preferably on the order of 11to 15 percent, and is tapered with drive walls 128. Thus, for example,if the widest part of the tooth 106 is 8 mm, the widest part of a sheave104 might be 11.25 mm, permitting a belt stretch of up to 3% where theteeth 106 can still be received in the sheaves 104. If the widest partof a sheave is 10 mm, the widest part of a tooth can be 7 mm in order topermit belt stretch of up to 3%. It is recognized that the belt will notstretch evenly, i.e., it will stretch more between the teeth thanincluding the teeth. This is because the belt is thinner between theteeth than at the teeth. Nevertheless, stretch here is measured overall,regardless of which portion of the belt is actually stretching underload. Also, the depth of each sheave 104 is greater than the height ofeach tooth 106 wherein each drive wall 128 is longer than acorresponding sidewall 126 so that the tooth will not bottom out in thesheave.

The belt pitch 112 when the belt 100 is at rest is less than thesprocket pitch 116. Thus, as illustrated in FIG. 2, a belt 100 under noload will effectively be driven in the stress zone 120 by the drive wall128 of a single sheave 104 acting against sidewall 126 of a single tooth106 received within the sheave. On the other hand, as illustrated inFIG. 3, a belt 100 under load P tends to stretch so that the belt pitch112, especially within the stress zone 120, more nearly equals thesprocket pitch 116. In this case, more teeth 106 are engaged bycorresponding sheaves 104. Under maximum stress, the belt pitch 112 willequal the sprocket pitch 116 as the belt 100 is pulled around thesprocket 102. In the embodiment illustrated in FIG. 3 wherein thesprocket 102 has ten sheaves 104, the maximum number of sheaves thatwill drive a corresponding tooth, at least in whole or in part, is six.Since the teeth 106 are smaller than the sheaves 104, and since the beltpitch 112 is smaller than the sprocket pitch 116 (below maximum load),the elasticity of the belt will permit anywhere from one to six teeth onthe belt to be driven by the sprocket 102. The higher the load on thebelt, the more teeth will be engaged by the sprocket.

Looking now at FIGS. 5 and 6, it can be seen that a belt 150 accordingto the invention can be used with existing sprockets. Here, the belt 150is shown installed on sprockets 152 and 153 of the type that can beobtained from Intralox, Inc. Sprocket 152 may be a drive sprocket, inwhich case sprocket 153 will be the slave sprocket. Each sprocket 152,153 has a number of transverse grooves or sheaves 154 adapted to receiveteeth 156 in driving engagement as described above. The relativedimensions of components in this embodiment will be as described abovefor the embodiment of FIGS. 2 and 3. In addition, there may be trackingtabs 158 adapted to engage corresponding slots 160 in the belt 150 tomaintain proper alignment of the belt.

Preferably, the teeth 106, 156 will be integral with the belt 100, 150.A method of making an endless belt according to the invention is shownin FIG. 7. An appropriate thermoplastic material such as polyester orpolyurethane is placed in an extruder 170. Using conventional extrusionmethods, a flat ribbon 172 of thermoplastic material is extruded fromthe extrusion nozzle 174. While the flat ribbon 172 is still soft andmalleable, it is passed over a profile drum 176. The profile drum 176has on its outer surface a plurality of grooves 178 corresponding inshape to the teeth 106, 156 to be formed. A compression drum or belt 180presses against the flat ribbon 172, compressing the ribbon, and forcinga portion of the flow into the grooves 178. Upon exiting the profiledrum 176, the formed ribbon 182 passes through a cooling station 184where it hardens. The temperature of the flat ribbon 172, the pressureon the ribbon 172 between the compression drum 180 and the profile drum176, and the cooling time are all established and determined toanticipate the final thickness of the belt, the dimensions of eachtooth, and the belt pitch. In this respect, it will be understood thatthere is a predetermined amount of shrinkage of the formed ribbon 182 asit cools. Upon cooling, the formed belt is collected and stored until itis needed to form an endless belt.

Another method of manufacturing the belt is to start with a homogeneousbelt of approximately 7 to 8 mm in thickness, and machine away materialbetween adjacent teeth 106, 156 to a depth of about 3 to 4 mm. Thismethod necessarily generates scrap.

Another method of manufacturing the belt is to start with a homogeneousbelt approximately 3–4 mm in thickness, and injection mold teeth at anappropriate belt pitch onto one surface of the belt. In this method, theteeth can be different material. For example, the belt can be formed ofpolyester such as COPE, with the teeth being formed of a urethane.

Another method of manufacturing the belt is to start with a homogeneousbelt approximately 3–4 mm in thickness, and friction mold teeth at anappropriate belt pitch onto one surface of the belt. In this method, theteeth can be different material. For example, the belt can be formed ofpolyester such as COPE, with the teeth being formed of a urethane. Ithas been found that applying a 160 Hz orbital motion of a polyestertooth on a polyester belt for three seconds creates enough softening forthe tooth to bond to the belt.

It will be understood that certain variations and modifications can bemade without departing from the scope of the invention. For example, thelength of each tooth need not extend to the edge of the belt asillustrated. A narrow rank of teeth may be sufficient. Further, a narrowbelt having a rank of teeth can be preformed and adhered or bonded to aninner surface of a larger belt. Moreover, if desired, coloring pigmentsand/or antibacterial agents can be added to the thermoplastic prior toextrusion.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation, and the scope of theappended claims should be construed as broadly as the prior art willpermit.

1. A thermoplastic conveyor belt segment having an outer surfacesubstantially free of discontinuities and an inner surface with aplurality of teeth extending therefrom at a given belt pitch andopposite ends, the belt segment being stretchable through its length,and wherein the teeth are integrally formed with the conveyor beltsegment by molding the teeth with the belt, whereby the belt segment canbe made to a selected length and the ends spliced together to form aconveyor belt of predetermined length.
 2. The thermoplastic conveyorbelt segment according to claim 1 wherein the molding comprises thesteps of: providing a flat ribbon of thermoplastic material; moldingteeth onto one surface of the flat ribbon; and curing the flat ribbonwith the teeth.
 3. The thermoplastic conveyor belt segment of claim 2wherein the teeth are injection molded.
 4. The thermoplastic conveyorbelt segment of claim 2 wherein the teeth are friction molded.
 5. Athermoplastic conveyor belt segment having an outer surfacesubstantially free of discontinuities and an inner surface with aplurality of teeth extending therefrom at a given belt pitch andopposite ends, the belt segment being stretchable through its length,and wherein the teeth are integrally formed with the conveyor beltsegment by machining away material from the inner surface betweenadjacent teeth, whereby the belt segment can be made to a selectedlength and the ends spliced together to form a coveyor belt ofpredetermined length.